A standard hammer drill has a drive spindle that is rotated about a spindle axis and a hammer displaceable axially in the drive spindle. A chuck is carried on the spindle so as to rotate therewith, and holds a drill bit having a shaft having a radially outwardly open and at least generally axially extending groove. The chuck holds the drill bit so it is centered on the spindle axis, and the hammer is effective on the rear end of the drill bit. The bit is rotationally coupled to the chuck body, which as mentioned above is rotated, and is at least limitedly axially reciprocated by the hammer so that it can drill through masonry or the like.
Since at least limited axial reciprocation is necessary for the drill bit, it is necessary that the bit not be solidly axially coupled to the chuck, as then the hammering would have to reciprocate the chuck also. Accordingly, the system of my copending U.S. patent application Ser. No. 591,975 filed March 21, 1984 describes a chuck used with a hammer drill having a drive spindle rotatable about a spindle axis and a hammer displaceable axially in the drive spindle, and with a drill bit having a shaft extending along a shaft axis and having a radially outwardly open and at least generally axially extending groove. The chuck has a generally tubular chuck body centered on a chuck axis and having a central axially extending passage. This body is normally carried on the spindle and receives the shaft of the drill bit in its passage with the spindle, shaft, and chuck axes all coaxial. The hammer projects axially forward into the passage and is engageable therein with the bit. A plurality of jaws radially but not angularly displaceable in the chuck body have inner ends engageable with the drill-bit shaft and in turn provided with formations complementarily engageable in the groove thereof. An actuator is connected to the jaws for moving same radially synchronously and thereby centering the bit in the body by engagement of the inner ends with the shaft and for engaging the formations in the grooves for rotationally linking the body and bit while permitting limited relative axial movement
Such an arrangement is somewhat complex. In addition it is not impossible for the jaws to move out somewhat during prolonged hammer drilling, so that the bit will wobble about and make a sloppy hole.